The present invention relates to an improved bearing design and more particularly to contouring the bearing surface to compensate for bending of the rotor under heavy loads.
Many industrial and marine, or shipboard, gearing applications require designs which are light weight and reduced in size even though the power transmitted is very high. For example, marine gearing often must be reduced in size and weight to accommodate ship weight targets as well as available machinery space. While marine applications, as described below, are generally speed reducers, industrial applications are either speed reducers or speed increasers. In ship propulsion applications, the gearing is provided for speed reducing, that is to reduce the speed of the relatively high speed engine to rotate the ships' propellers at much lower speeds. Such applications typically transmit in excess of 25,000 shaft horsepower, wherein the input speed may be in the order of 3000 RPM with a reduced propeller speed of around 125 RPM. Each pinion and gear in the gear train is supported on a pair of babbitted journal bearings.
The reductions in size and weight of the gear train or gearing increases the loading density on the gearing elements, resulting in smaller rotor diameters with increased face loading as well as higher bearing operating pressures. Contact stresses in the order of 70,000 PSI (pounds per square inch) and bending stress in the order of 30,000 PSI are encountered. During operation, such rotors experience increased bending deflections due to the reduced bending inertia of the cross section of the rotor and higher loading on the gear face. The larger bending deflections over the length of the journal bearings result in reduced operating oil film thicknesses at the edge of the bearing closest to the rotor face.
One method of accommodating the resultant rotor slope across the bearing surface and obtain adequate film thickness is to use tilting pad journal bearings which align themselves with the bending line of the rotor. However, tilting pads add to the complexity and cost of the gearing as well as higher power losses, thereby reducing the overall efficiency of the machine. Accordingly, it is desirable to accommodate for the bending deflections without the use of tilting pads.
Also, it is desirable to be able to accommodate rotor bending in a number of different types of bearings such as pressure groove bearings, three lobe bearings and elliptical bearings, which are used in reduction gears, often in the same gear set.